This invention relates to electroless deposition of copper onto a styrene-derivative polymer using a vacuum deposited copper catalyst. More particularly, this invention relates to ultraviolet (UV) light treatment of the styrene-derivative polymer surface prior to vacuum deposition of a discontinuous thin copper film that renders the film active during subsequent electroless copper deposition. In one aspect of this invention, the ultraviolet light treatment is applied to a selected region of the polymer surface to produce a plate having a desired pattern.
A metal plate, such as copper or nickel plate, may be deposited onto a nonconductive polymer surface by electroless deposition. The plate is useful, for example, in the manufacture of a microelectronic circuit pattern. The plate may be applied and etched to remove unwanted metal and define the pattern. However, this necessitates a mask, such as a photolithographic mask, temporarily applied to the surface to protect the pattern from the etching media. Alternately, a mask may be applied to the surface prior to plating. In either case, additional steps are required to apply, develop and remove the mask. Thus, it is desired to avoid the steps for forming a mask on the polymer, while depositing the plate only onto areas that produce the pattern.
Prior to electroless deposition, a catalyst is applied to the surface. A common catalyst is formed by an aqueous colloidal suspension of tin and palladium particles. Alternately, it is known that a discontinuous thin film of copper may be suitable as a catalyst in some applications. However, for a substrate formed of a styrene-derivative polymer, such as polystyrene, acrylonitrile-butadiene-styrene (ABS) copolymer or poly(styrene-acrylonitrile) (SAN), vacuum-deposited copper is ineffective for catalyzing electroless deposition without pretreatment of the polymer surface. Wet chemistry treatments conventionally used with palladium-tin catalyst require masking to treat only the pattern regions and are therefore not desirable. I have found that exposure of a styrene-derivative polymer surface to ultraviolet light prior to vacuum deposition of a thin copper film produces a film on the exposed areas that is active for catalyzing electroless copper deposition, whereas the film applied to unexposed regions is not. This is surprising in part because a similar ultraviolet light treatment is not effective for activating a palladium-tin catalyst.
It is an object of my invention to provide an improved method for electroless deposition of a metal plate onto a styrene-derivative polymer surface using a vacuum-deposited metal catalyst, which method comprises ultraviolet irradiation of the styrene-derivative polymer surface prior to catalyst deposition to activate the catalyst for subsequent electroless deposition.
More particularly, it is an object of this invention to provide an improved method for applying an electroless copper plate onto selected surface regions of a styrene-derivative polymer, which method comprises exposing the regions to ultraviolet light and vacuum depositing a discontinuous thin copper film onto the entire surface, including both exposed and unexposed regions, whereupon the film is rendered active on exposed surfaces for catalyzing subsequent electroless deposition, but is inactive on unexposed surfaces. Thus, this ultaviolet light exposure may be conveniently controlled to restrict the metal plate to a desired pattern on the product surface. The plate is advantageously deposited using but a single wet chemistry step, that of electroless deposition, and without requiring additional steps for coating and developing a protective mask on the surface.